Support configurable default font and size

Add default_font and default_size options to Document.create, rendered
as w:docDefaults in styles.xml. Defaults to Arial 12pt. Adjust heading
sizes for better visual hierarchy.

Styling based on uutilsynet guidance for accessible documents:
https://www.uutilsynet.no/veiledning/rettleiar-universelt-utforma-word-og-pdf-dokument/1636

lib/notare/package.rb

@@ -59,7 +59,12 @@ module Notare
     end
 
     def styles_xml
-      Xml::StylesXml.new(@document.styles, @document.table_styles).to_xml
+      Xml::StylesXml.new(
+        @document.styles,
+        @document.table_styles,
+        default_font: @document.default_font,
+        default_size: @document.default_size
+      ).to_xml
     end
 
     def numbering_xml